This invention relates to the application of a particle-embedded coating to a substrate and, more particularly, to the application of an abrasive coating to the tip of a gas turbine blade.
In an aircraft gas turbine (et) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The turbine blades are mounted on a turbine disk, which rotates on a shaft inside a tunnel defined by a cylindrical structure termed the stationary shroud. The hot combustion gases flow from the engine""s combustor and into the tunnel. The hot combustion gases pass through the turbine blade structure and cause it to turn. To achieve a high efficiency, it is important to minimize the leakage of the hot combustion gases around the turbine. However, the sealing of the turbine structure against such leakage presents a problem, because the components of the structure expand and contract differently during the temperature changes of over 2000xc2x0 F. that are experienced during each cycle of engine operation.
To prevent the leakage of hot combustion gases around the turbine, it is known to size the components so that the tips of the turbine blades extend to a slightly larger diameter than the inside diameter of the shroud, when the engine is operated. Upon initial startup and break-in of the engine, the tips of the turbine blades contact the inside of the shroud, and wear away a path along a circumferential track around the inside surface of the shroud. Further contacting between the blade tips and the stationary shroud sometimes occurs again during operation of the engine in the usual operating conditions or under unusual conditions such as application of emergency power or large loads applied to the components.
In these circumstances, it is desired that material is worn away from the stationary shroud by the turbine blade tips, rather than vice versa. To ensure that the blade tips are more abrasive than the shroud material, it is known to apply a particle-entrapped abrasive coating onto the tips of the turbine blades. The processing of the blade tips to add the |0 particle-entrapped coating increases the cost of the manufacture of the turbine blades. Additionally, the existing particle-entrapped abrasive coatings are sometimes lacking in abrasiveness, durability, and service life.
There is accordingly a need for an improved approach to the preparation of abrasive tips on gas turbine blades, and other applications having some of the same requirements. The present invention fulfills this need, and further provides related advantages.
This invention provides an approach for applying particle-embedded coatings to articles such as turbine blades, and articles made by the approach. The present technique produces an improved particle-embedded coating material, and also reduces the process costs of preparing the articles as compared with prior approaches. The present approach is compatible with the other processing used for turbine blades, such as the application of environmental coatings and thermal barrier coatings on the airfoil surface of the turbine blade, and with the use of modified coatings such as platinum aluminides.
A method for coating an article comprises the steps of furnishing an article substrate, thereafter applying a particle-entrapped coating to the article substrate, and thereafter applying an aluminum-containing coating overlying the particle-entrapped coating. The step of applying the aluminum-containing coating includes the steps of providing a source of aluminum contacting the article substrate that deposits aluminum onto the article substrate at a coating temperature, and heating the article substrate to the coating temperature so that an aluminum coating is deposited onto the article substrate overlying the particle-entrapped coating. The aluminum coating and the particle-entrapped coating are interdiffused with the article substrate. The step of applying the aluminum-containing coating occurs without substantial prior interdiffusing of the particle-entrapped coating with the article substrate as a separate step.
In this approach, the step of applying a particle-entrapped coating desirably includes the step of applying a coating comprising boron nitride particles (preferably cubic boron nitride particles) embedded in a matrix comprising nickel. The step of providing a source of aluminum desirably includes the step of providing a gaseous source of aluminum, preferably AlF3. The coating temperature is greater than about 1800xc2x0 F., preferably from about 1800xc2x0 F. to about 2000xc2x0 F., and most preferably from about 1925xc2x0 F. to about 1975xc2x0 F.
In another embodiment, a method for coating an article substrate comprises the steps of furnishing an article substrate having a first region and a second region, thereafter applying a particle-entrapped coating to the first region, and thereafter applying an aluminum-containing coating to the first region and to the second region. The step of applying the aluminum-containing coating includes the steps of providing a source of aluminum contacting the article substrate that deposits aluminum onto the article substrate at a coating temperature, and heating the article substrate to the coating temperature so that an aluminum coating is deposited onto the first region and onto the second region, and so that the aluminum coating and the particle-entrapped coating are diffused into the article substrate. The step of applying the aluminum-containing coating occurs without substantial prior interdiffusing of the particle-entrapped coating with the first region as a separate step.
In the application currently of most interest, a method for coating a turbine blade comprises the steps of furnishing a turbine blade substrate having an airfoil and a tip at an end of the airfoil, thereafter applying a particle-entrapped tip coating to the tip of the airfoil, and thereafter applying an aluminum-containing coating to the airfoil, including to the tip of the airfoil overlying the particle-entrapped tip coating as well as the rest of the airfoil. The step of applying the aluminum-containing coating includes the steps of providing a source of aluminum contacting the airfoil that deposits aluminum onto the airfoil at a coating temperature, and heating the airfoil to the coating temperature so that an aluminum coating is deposited onto the airfoil, and so that the aluminum coating and the particle-entrapped tip coating are diffused into the turbine blade substrate. The step of applying the aluminum-containing coating occurs without substantial prior interdiffusing of the particle-entrapped tip coating with the tip of the airfoil as a separate step.
The present approach produces a particle-entrapped coating that has improved performance as compared with conventional particle-entrapped coatings. In the case where the as-deposited particle-entrapped coating is boron nitride particles in a nickel or nickel-alloy matrix, the concurrent interdiffusing of aluminum into the as-deposited particle-entrapped material and the interdiffusing and bonding of the matrix to the substrate produces an altered structure of particles that are more abrasive, harder, more durable, and longer-lived than conventional boron nitride particles. The present approach has the additional advantage that it shortens the processing time and reduces the processing cost of the article by eliminating a separate step of interdiffusing the particle-entrapped coating with the article substrate as a separate step.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.